1. Field of the Invention
This invention relates to a low pollution type coating composition having improved solubility in an organic solvent and resistance to color change of resulting coated film and using an acid catalyst and a non-aqueous polymer dispersion having excellent dispersion stability and compatibility with a coating resin.
2. Related Art Statement
Recently, needs for higher solid coating compositions are raised for reducing a volatile organic compounds and improving a quality of coated film appearance. In general, it is known to use low viscosity and high solid materials, that is, a low molecular weight base resin, a curing agent and a non-aqueous polymer dispersion for increasing the solid content of the coating. Among these materials, the non-aqueous polymer dispersion itself is low in the viscosity and high in the solid content and has a function for controlling fluidity of the coating such as prevention of sagging in vertical direction, uniform orientation of metallic or inorganic flat pigment or the like, so that it is particularly regarded as important for developing the higher solid coating. In this case, the function for controlling the fluidity of the coating is effective as the particle size of the non-aqueous polymer dispersion becomes smaller.
In the conventionally practised non-aqueous polymer dispersion, a so-called graft copolymer or a high molecular weight melamine resin having a long side chain of low polarity is frequently used as a dispersion stabilizer in order to stably disperse particles into an organic solvent.
The mechanism of stabilizing particles through such a dispersion stabilizer is considered due to the fact that a part of the molecular structure of the dispersion stabilizer is fixed to the surface of the particle and the remaining portion thereof is solvated by the organic solvent to prevent agglomeration between the particles.
Therefore, the non-aqueous polymer dispersion must be prepared so that the particle is insoluble in the organic solvent but the dispersion stabilizer is soluble therein.
That is, the non-aqueous polymer dispersion is composed by properly balancing polarity among particle, dispersion stabilizer and organic solvent. For this end, if this balance is destroyed, the settlement due to particle agglomeration or the increase of viscosity due to particle dissolution or gellation occurs. As the balance among the three components, there are considered the following two relations (A) and (B):
______________________________________ (A) (B) ______________________________________ Polarity of particle high low Polarity of dispersion stabilizer low high Polarity of organic solvent low high ______________________________________
In the relation (B), vinyl type monomer forming particles must have a low polarity. However, since the glass transition temperature is usually low in the polymerization of such a low polarity vinyl type monomer, the particles made from the vinyl type monomer become fairly soft. On the other hand, the synthesis of the non-aqueous polymer dispersion is usually carried out by radical polymerization, so that the temperature in the synthesis is frequently more than about 60.degree. C. Therefore, the relation (B) is not so favorable because there is a high possibility of causing the fusing between particles in the synthesis of the non-aqueous polymer dispersion.
On the contrary, the relation (A) does not cause the problem as mentioned above and is favorable. In order to obtain a stable non-aqueous polymer dispersion, therefore, it is desirable that the polarity of particle is higher and the polarities of dispersion stabilizer and organic solvent are lower. Many of the conventional non-aqueous polymer dispersions are synthesized on the basis of this technical idea, and in this case, the polarity of the dispersion stabilizer is made considerably low.
For instance, a method of synthesizing a non-aqueous polymer dispersion by using a graft copolymer composed mainly of methyl methacrylate and having 5 mol condensed structure of 12-hydroxy stearic acid in its side chain as a dispersion stabilizer is disclosed in Japanese Patent Application Publication No. 43-16,147 and Japanese Patent laid open No. 54-150,439. According to this method, the polarities of the dispersion stabilizer and organic solvent are considerably low, so that even when the ratio of particle to dispersion stabilizer is high, the non-aqueous polymer dispersion having a small particle size can be obtained stably.
On the other hand, when the high molecular weight melamine resin is used as the dispersion stabilizer, the effect of stabilizing dispersion of particles is small as compared with the case of using the graft copolymer. Therefore, this case is suitable for the synthesis at a low level of particle concentration, wherein the particle size is large.
Therefore, in order to effectively develop the fluidity controlling function for the coating and the increase of solid content at a smaller amount of the dispersion stabilizer, it is considered that the use of the non-aqueous polymer dispersion containing the graft copolymer as the dispersion stabilizer is suitable.
As a curing agent, low molecular weight alkyletherified melamine resin is used from viewpoints of storage stability of coating, pot life, toxicity, handling easiness, cost and the like, while low molecular weight polyol resin is often used as a base resin.
In order to bake and cure the higher solid coating comprised of the above components, it is usually required to use an acid catalyst for promoting ether exchange reaction between the alkyletherified melamine resin and the low molecular weight polyol resin.
As the acid catalyst, there have hitherto been known aromatic sulfonic acids such as P-toluene sulfonic acid, dodecyl benzenesulfonic acid, dinonyl naphthalenesulfonic acid, dinonyl naphthalenedisulfonic acid and so on (Paint & Resin, pp26-28, March/April, 1982). These aromatic sulfonic acids are used in form of a salt blocked with a tertiary amine or an ester with an epoxy compound for improving the storage stability of the coating as disclosed in Japanese Patent laid open No. 52-127,927.
In the conventional non-aqueous polymer dispersions, however, since the polarity of the dispersion stabilizer is set to a low value as previously mentioned, when such a dispersion stabilizer is mixed with a relatively high polarity resin such as thermosetting resin for ordinary coatings, the compatibility becomes insufficient, so that the particle agglomeration is caused or the muddiness or delustering is caused in the cured film.
For example, when the non-aqueous polymer dispersion produced by the method as disclosed in Japanese Patent Application Publication No. 43-16,147 and Japanese Patent laid open No. 54-150,439 is mixed with a relatively high polarity resin such as an acrylic resin having a high acid value or hydroxyl value, a short oil type alkyd resin or the like, since the polarity of the dispersion stabilizer is too low, the compatibility with the resin having a relatively high polarity becomes insufficient, so that the pigment agglomeration and the like are caused to easily injure the appearance of the coated film.
As the acid catalyst, the conventional aromatic sulfonic acid has a difficult point in the solubility in organic solvent, so that when the aromatic sulfonic acid is particularly used in form of a salt blocked with a tertiary amine, it may be crystallized in the coating to form bittiness in the coated film. On the other hand, when the aromatic sulfonic acid is used in form of an ester with an epoxy compound, the solubility is improved. In the latter case, however, since the dissociation efficiency to original aromatic sulfonic acid is low, the acid catalyst is required to be used in a large amount, so that a large amount of the acid catalyst remains in the coating, resulting in the occurrence of color change in the coated film.